This invention relates to a driving device for driving a semiconductor laser to emit laser light, and more particularly to a driving device capable of modulating light intensity to be outputted from a semiconductor laser.
A semiconductor laser has been conventionally utilized in an image forming apparatus such as a laser printer or the like. In order to increase an image forming speed in such an image forming apparatus, it is really preferable to set the maximum light intensity to be used for an exposing process approximately to the maximum permissible light intensity of the semiconductor laser. However, an output characteristic of the semiconductor laser is unavoidably changed by an external factor such as temperature or the like, and thus it is difficult to perform the above real setting operation in light intensity.
In view of the above disadvantage of the semiconductor laser, the same applicant as this application has proposed a driving device for dynamically changing a biasing current for stimulating light emission of the semiconductor laser in U.S. Pat. No. 4,916,706 published on Apr. 10, 1990, and this device is shown in FIG. 1.
In this type of driving device, the semiconductor laser 1 is pumped by a pumping or biasing current corresponding to the maximum value of modulated image data on the basis of a light-intensity correction signal which occurs every constant period. The intensity of laser light outputted from the semiconductor laser 1 is compared with the permissible light-intensity of the semiconductor laser 1 in a judging circuit 2 to output a result to a threshold data generator 3. The threshold data generator 3 generates, for example, an 8-bit threshold data on the basis of the inputted result thereto, and then outputs the threshold data to a adding circuit 4. In the adding circuit 4, the 8-bit threshold data and 8-bit modulated data are added to each other, and then inputted to a driving circuit 5 for driving the semiconductor laser 1. The driving circuit 5 generates a biasing current corresponding to the added 8-bit data to control light-emission of the semiconductor laser with the biasing current.
However, since many semiconductor lasers which have been commercially sold, have in a market have individually different characteristics such as a threshold current and so on, it frequently occurs that some semiconductor lasers are inoperable for the above driving device. Therefore, it has been hitherto required to select semiconductor lasers in consideration of not only the least upper limit biasing current, but also the change of a threshold current in accordance with the change of temperature, that is, the change of the lowest limit of an operating area which corresponds to the lowest biasing current for maintaining the linearity between an output power of the laser light and a supplied current to the semiconductor laser. This selection is very intricate and thus disturbs a manufacturing cost of the semiconductor laser to be lowered.
In order to overcome the above disadvantage, it is possible to expand a changeable range of the threshold data to be generated in the threshold data generator 3 in accordance with variation of the threshold biasing current of the semiconductor laser. However, this method causes a large quantization error because the changeable range of the threshold data which is represented by one binary code is also broader, so that it is difficult to finely and accurately change the output power of the laser light of the semiconductor laser. Particularly, in a case where such a driving device is used in an image forming apparatus such as a laser printer, an output image is deteriorated in color gradation, and thus an image having intermediate color gradation (tone) can not be obtained.